Since the cash inflows are receivable in advance, the flows are from time 0 to time 4.
So the present value of them is (12,000 + (12,000 x the 4 year annuity factor at 10%) = 50,040. The present value of the 15,000 in 5 years time = 15,000 x 0.621 = 9315 (and this is negative because it is a cost)
So the NPV = 50,040 – 9315 – 40,000 = + 685
Surely there are workings in whichever book you found the question? (If not then you should be using a Revision Kit from one of the ACCA approved publishers 🙂 )